Ink jet printers are becoming an increasingly popular type of device for recording permanent images on paper. Ink jet printers operate by directing a stream of minute ink droplets at the paper so as to produce a distinct pattern of individual ink dots thereon. The final image produced is the collective form of the individual ink dots. By forming ink dots at selected locations on the paper, and by regulating the number of ink dots formed on the paper, an ink jet printer can be used to create almost any type of print: text; graphics; or images. This capability has made it attractive to attach ink jet printers to computer systems that produce figurative, image and textual output simultaneously. This is because a properly programmed ink jet printer can be used to produce a complicated figure, and a detailed description of the figure, or the same page or the inclusion of images in combination with the above or alone.
Moreover, many ink jet printers are capable of discharging multiple colors of ink so as to generate quality color figures and illustrations. This capability has contributed to their popularity since computers that can generate multi-color video output in the form of figures and images are becoming increasingly common. These computer systems require printing devices that can produce permanent images of the output they generate.
Ink jet printers are provided with an inking system that includes one or more ink jets that are directed towards the paper on which the ink is to be deposited. Each ink jet typically has a jet opening through which the ink is discharged and a jetting chamber immediately behind the jet opening. A transducer generates vibrational movement to the ink in the jetting chamber to provide the mechanical energy needed to discharge the ink droplets therefrom. A feed line from an ink reservoir supplies ink to the jetting chamber for discharge.
A variety of inks are used in ink jet printers, including inks that are normally liquid at room temperatures and above (hereinafter referred to simply as "liquid" inks) and those that are normally solid at room temperatures but that are heated to elevated temperatures to liquefy them for jetting (the so-called "hot-melt" or "phase-change" inks). Hot-melt inking systems are used, in part, because the ink they discharge solidifies rapidly on contact with the paper and the forms ink dots with very sharp optical edges so the resulting images are of very high quality. Hot melt inks also have exceptional true color mixing properties which is an important characteristic for color printers that typically have three base color inks, plus black, that are blended together to print a very large spectrum of intermediate colors.
Practically every ink jet printer, liquid and hot melt, must be able to consistently discharge ink droplets of substantially the same size. This is so the droplets form identical ink dots on the paper which are necessary to produce a final image that is an accurate representation of the desired output. Another consideration in the design of an ink jet printer is insuring the ink droplets discharged from the jets travel at a substantially identical velocity. This is important because with most ink jet printers either the print head carry the jets or the paper is moving. Consequently, if the ink droplets must travel at a velocity so they are all accurately deposited on the paper to form the desired image. If the ink droplets travel at varying velocities they will be inaccurately deposited on the paper and the quality of the resulting image will be degraded.
In order for most ink jet printers to discharge ink droplets of substantially the same size, it is necessary for each of the printers' jets to have a full head of ink through the jetting chamber, up into the jet opening. If there is any air in the jetting chambers, typically in the form of small air bubbles, when the jets are activated they discharge air. Alternatively, air in the jetting chambers distorts the velocity at which the ink droplets are discharge from the jets so that consequently the droplets inaccurately form ink dots on the paper. Air bubbles of 10 mils (0.01 inch) in diameter in a jetting chamber have been found to prevent the discharge of ink droplets therefrom. Smaller air bubbles are suspected of distorting the velocity of ink droplets that are discharged from the jets.
Sometime during the operation of almost every ink jet printer, one or more of the ink jets lose their full head of ink. This often happens at the end of the day after the printer has been turned off, and the ink jets are no longer being activated to discharge ink droplets.
Air bubbles are prone to form in almost every type of ink jet printer, liquid or hot melt. Air bubbles form in a printer employing liquid ink as a consequence of very minute gas bubbles in the ink agglomerating into large bubbles in the jetting chambers. Also, when the printer is turned off, such as at the end of the day when printing is complete, the ink menisci at the jet opening have a tendency to dry out and break. This allows air to flow through the jet opening and enter the jetting chamber. Furthermore, shock and vibration an ink jet printer may be subjected to in a normal environment may be sufficient to cause bubbles to form within the ink jets.
Air bubbles form in hot melt printers as a consequence of the printer being turned off and on such as at the end and start of successive days of using the printer. When a hot melt printer is turned off, the heating elements are deactivated. Ink in the jets then resolidifies and contracts. When the printer is turned on again and the heating elements are reactivated, the reliquefied ink is in the form of a bubbly froth head unsuitable for accurate jetting.
Moreover, the capability of most ink jet printers are adversely affected by contaminants such as small bubbles and bits of matter that form in the interior spaces of their jets. These contaminants disrupt the flow of ink to the jetting chamber and out the jet opening so that droplets of varying size are discharged therefrom. In some instances these contaminants may even block the flow of ink through the jets resulting in the cessation of the discharge of droplets therefrom. In either situation the result is the same, ink droplets are inaccurately deposited on the paper being printed on, diminishing the quality of the final image.
Many ink jet printers are provided with a priming system for forcing a full head of ink to its jetting chambers and for flushing contaminants out of the jets. Typically, priming systems operate by applying pressure to a reservoir where the ink is stored prior to its discharge through the jets. The pressure forces ink through the jets up to the jet openings so as to fill the jets with ink to insure proper performance, including the consistent discharge of ink droplets of substantially identical size whenever the ink jets are activated.
Many priming systems operate by supplying pressurized air to the ink reservoir. The air forces the ink in the reservoir through the ink jets.
Current priming systems typically have a valve or seal used to close a first opening through which ink is supplied to the inking system. After the first opening is closed, air is introduced into the reservoir through a second opening which may have its own valve or seal. After priming is completed, the valves or seals associated with both openings must be set in the appropriate positions so that they do not interfere with the supply of ink to the reservoir or with the printing process.
These priming systems have a number of disadvantages. These systems usually have a large number of small moving parts which must work properly, and in the proper sequence, for priming to be successful. Moreover, the large number of parts these priming systems have occupy a significant amount of space and thus make it difficult to reduce the overall size of the ink jet printer. These priming systems also tend to be complex to install and expensive to manufacture.
Another disadvantage associated with current priming systems is that they are often an integral part of the reservoir and associated ink jets they are designed to prime. Color ink jet printers usually have one reservoir and set of jets for each color of ink discharged. Thus, to provide a color printer with a complete priming system, it is necessary to provide an individual priming system for each reservoir and jet assembly. This adds to the overall complexity of the printer, the space the priming system occupies, the possibility that the priming system will malfunction, and the cost of the priming system.
Still another disadvantage of current priming systems is that they sometimes leave the jets they are intended to prime with a bubbly froth head. This happens because the ink primed through the jets travels at a very high velocity. Consequently there is a significant amount of turbulence at the head of flow and when it reaches the reduced diameter of the jet opening, turbulence leaves bubbles in the jetting chamber. Furthermore, the walls of many ink jets are often not sufficiently "wetted" prior to priming of the jets. As a result the high velocity priming ink tends to travel in the center of the jets, away from the wall. This is another source of turbulence during the priming flow that sometimes causes bubbles to remain after priming is completed. In either case, the bubbles that remain in the jetting chamber after the priming process defeat the purpose of the prime.
A further problem with many ink jet priming systems is that they do not consistently "stream" ink discharged during the priming process. Streaming is the discharge of ink at or above a sufficient velocity so that it is deposited in the appropriate waste container. Many ink jet priming systems cause the ink to trickle out of the ink jets at either the beginning or end of the priming discharge. Ink that so trickles out of the ink jets can dirty the face of the jets possibly distorting the discharge of the ink drops or smearing paper that is passed adjacent thereto. Alternatively, ink that trickles out of the ink jets can flow down the printer, dirtying the entire printer assembly and possibly fouling the printer electronics or working components.
Hot melt ink jet printers have a special problem with ink that trickles out the ink jets. Eventually the ink hardens into a solid mass that if not removed, almost always adversely affects the printer's operation.
To date, few ink jet priming systems have found a way to eliminate problems associated with ink trickle during the priming process. Some priming systems simply have brushes or other wiping mechanisms to clean trickled ink off the face of the jets after priming.
Moreover, many current ink jet priming systems have one or more subsystems that require operator assistance to properly function. For instance, some priming systems have hand-operated pumps system to supply the pressurized air and ink needed to prime the jets. Other priming systems may require the user to set one or more of the valves and seals needed to control the flow of air and ink through the jets. These systems suffer the inefficiencies of human operation.
Furthermore, over time, the operator manipulated parts of these priming systems may become covered with ink or other dirt so as to make priming the printer an unpleasant task. Also, all ink jet priming systems generate waste ink which must be collected for removal. Some priming systems are provided with a waste ink collection and removal subsystems that makes the removal of the waste ink an inherently dirty task. Thus, in some situations the tasks required for the operator to prime the printer may be considered so unpleasant that priming is performed carelessly, or not at all.